A 22 year-old woman with tyrosinemia type I (HT1) married her first cousin who is heterozygous for the same FAH mutation for which the patient is homozygous. During her pregnancy she was treated with diet (prescribed tyrosine intake 300 mg/day), and nitisinone (60 mg/day). Median plasma tyrosine levels were 560 μmol/L (range: 375-838, n = 21) and nitisinone 51 μmol/L (range: 41-57, n = 3) during pregnancy. She gave birth to a clinically healthy girl affected with tyrosinemia type 1. Birth was normal (birth weight 2615 g) and the baby had normal liver function, normal plasma alpha-fetoprotein concentrations, low urinary excretion of phenolic acids and no detectable succinylacetone. At birth, the baby had hypertyrosinemia (860 μmol/L in blood cord) and nitisinone levels of 14 μmol/L. Following molecular confirmation of the diagnosis of HT1 specific treatment began on day 15 by which time she had detectable urinary succinylacetone.